Thinking of god is always possible. But physical contact with sacred things is forbidden.
The reason is given in Bhagwat Purana, (Canto 6 chapter 9)
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4. Once upon a time, however, the King of heaven, Indra, understood that Visvarupa was secretly cheating the demigods by offering oblations on behalf of the demons. He became extremely afraid of being defeated by the demons, and in great anger at Visvarupa he cut Visvarupa's three heads from his shoulders.
5. Thereafter, the head meant for drinking soma-rasa was transformed into a kapinjala [francolin partridge]. Similarly, the head meant for drinking wine was transformed into a kalavinka [sparrow], and the head meant for eating food became a tittiri [common partridge].
6. Although Indra was so powerful that he could neutralize the sinful reactions for killing a brahmana, he repentantly accepted the burden of these reactions with folded hands. He suffered for one year, and then to purify himself he distributed the reactions for this sinful killing among the earth, water, trees and women.
7. In return for King Indra's benediction that ditches in the earth would be filled automatically, the land accepted one fourth of the sinful reactions for killing a brahmana. Because of those sinful reactions, we find many deserts on the surface of the earth.
8. In return for Indra's benediction that their branches and twigs would grow back when trimmed, the trees accepted one fourth of the reactions for killing a brahmana. These reactions are visible in the flowing of sap from trees. [Therefore one is forbidden to drink this sap.]
9. In return for Lord Indra's benediction that they would be able to enjoy lusty desires continuously, even during pregnancy for as long as sex is not injurious to the embryo, women accepted one fourth of the sinful reactions. As a result of those reactions, women manifest the signs of menstruation every month.
10. And in return for King Indra's benediction that water would increase the volume of other substances with which it was mixed, water accepted one fourth of the sinful reactions. Therefore there are bubbles and foam in water. When one collects water, these should be avoided.
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